Subterranean “sliding” drilling operations typically involve rotating a drill bit on a downhole motor at a remote end of a drill pipe string. Drilling fluid forced through the drill pipe rotates the motor and bit. The assembly is directed or “steered” from a vertical drill path in any number of directions, allowing the operator to guide the wellbore to desired underground locations. For example, to recover an underground hydrocarbon deposit, the operator may drill a vertical well to a point above the reservoir and then steer the wellbore to drill a deflected or “directional” well that penetrates the deposit. The well may pass through the deposit at a non-vertical angle, e.g. horizontally. Friction between the drill string and the bore generally increases as a function of the horizontal component of the bore, and slows drilling by reducing the force that pushes the bit into new formations.
Current approaches measure weight on bit using a hookload signal at the surface during drilling operations. For drilling of vertical wells, assuming no buckling is occurring along the drill pipe downhole, the calculation of the downhole weight on bit is a straight forward one. If, however, the well is a directional well, such as during “sliding” drilling operations, then this approach to calculating weight on bit is not reliable. Once the drill bit kicks off the curve, the weight on bit displayed to the driller in current approaches is not the true weight on bit. Instead, in directional sections the driller depends on mud motor differential pressure to estimate the weight on bit. A challenge arises, however, because the mud motor differential pressure does not identify when the bit has exceeded its physical load limit.
Several additional challenges exist with the current uses of mud motor differential pressure. The mud motor pressure, which increases with weight on bit, is difficult to isolate from the internal pressure measurement (which includes annulus pressure drop, bit pressure drop, motor pressure drop, measurement while drilling pressure drop, and drill string pressure drop components). Though it is assumed when estimating mud motor differential pressure that the pressure drop across the mud motor is zero when the bit is off bottom downhole, that is not always the case. When a steerable assembly is in the hole, the bit may contact the side wall of the hole and cause reactive torque at the mud motor. When zeroing the differential while the bit is off bottom, this load (from the reactive torque) is removed as well, such that any pressure increase seen as going to bottom of the hole does not include this already-existing load on the mud motor.
Autodrillers typically use the current weight on bit estimation from the hookload signal during vertical drilling to keep a constant load on the drill bit. In directional drilling, however, the weight on bit estimate is not used because current approaches result in a weight on bit estimation that is not correct during directional drilling. Further, weight on bit estimates are currently used in mechanical specific energy (MSE) calculations, though they are not correct during directional drilling. As a result, the MSE calculations are likewise not correct during directional drilling. Another variable that is often poorly estimated is torque on bit, which currently is estimated based on top drive torque.
Though the current weight on bit may be used in autodrillers, problems arise when hookload is used for determining weight on bit. This is because the use of hookload relies upon the assumption that, as the drill string is lowered and touches bottom, the observed difference in hookload is all transferred to the bit at bottom. In reality, (for example in long lateral wells), frictional forces at various sticking points along the wellbore causes the drill pipe to bend as the drill string is lowered and a portion of the lost weight at the hook is supported by the bottom side of the horizontal hole and not the end of the hole where the bit is located. At the surface, this is measured as a lowering of the drill string and a reduction of hookload, though not all of the reduced hookload is transferred to the bit at bottom. At some point, the load on the sticking points in the wellbore is high enough that it overcomes the frictional forces. The drill string slips lower as a result, causing more of the weight to transfer to the bit. Such spikes in the downhole weight on bit can unnecessarily damage downhole equipment.
The present disclosure is directed to systems, devices, and methods that overcome one or more of the shortcomings of the prior art.